Separation anxiety has long been recognized as an anxiety-related disorder in dogs that is only observed in the owner's real absence or perceived absence. Over the past four decades, this condition has been the most commonly discussed disorder in published studies, but etiology, treatment, and prevention remain elusive. A review of the literature indicates a lack of comparative studies. Little progress has been made to build a new understanding beyond the clinical symptomology. Future research employing more rigorous designs and systematically building upon a clearly defined research questions is needed to advance our knowledge of this most common behavioral problem in dogs.